H.A Hepburn amplified the above Heinrich’s theory and arrived at the principle that an injury accident is the result of the convergence at the same point of time of 4 factors (1) Unsafe actionable (2) Unsafe conditional (3) Proximate casual and (4) Personal.
Here unsafe actionable and conditional factors are as usual Personal factor means person injured or likely to be injured by an accident and the person causing the accident. The proximate factor is that immediate causative factor such as failure of a break, sudden exposure to gas etc, which by its reaction causes a sudden dosing together or convergence of all the four factors to cause an injury accident. He emphasises that the four factors are complementary to one another in causation of any injury-accident such that, if any one or more can be withdrawn by any means during or just before convergence, an injury-accident can be prevented. The event of an accident will not be prevented by efforts to control any one of the factors to the exclusion of the others. Remedial measures must be adopted for each of the factors. Like Heinrich he also suggested planning and organising to prevent unsafe actions and remove unsafe mechanical or physical conditions.